This invention relates to a method and apparatus for determining the amount of radon in a gaseous sample, and more particularly to the direct on-site collection and measurement of radon emanating, for example, from a mine wall.
Radon is a chemically inert, radioactive, gaseous element produced in the disintegration of radium. In uranium mines, for example, radon emanates from the walls, and once in the air it decays with a 3.8-day half life to polonium-218, which in turn decays to lead-214, bismuth-214, polonium-214 and then to lead-210. These non-gaseous decay products, or daughters, are adsorbed by dust particles in the air. Clearly, excessive amounts of radon present physiological hazards. To reduce this health hazard, a large volume of air is continually forced into the mine to dilute the concentration of radioactive material.
Certain areas in the mine give off more radon than others. To detect these areas of concentration, or hot spots, and to determine the overall level of radon in the mine, a radon measuring device is used. In view of the limited space in a typical mine, a compact, portable radon measuring instrument is desirable. Since radon decays by alpha particle emission, with less than one percent gamma particle emission, an alpha particle detector must be used for the most efficient radon measurement. The high gamma particle background in an uranium mine also makes a gamma particle detector less useful for measuring low levels of radon.
There are a number of problems associated with measuring radon emanating from the wall of a mine. The mine air containing radon must be excluded from the sample taken from the wall. The radon daughters are alpha particle emitters and have to be removed from the sample. Beta particles and gamma radiation from the various radioactive nuclides cannot be allowed to interfere with the radon measurement.
Several types of detectors for alpha particles exist, including scintillant and phototube detectors, solid state detectors, and gas proportional detectors. In the scintillant-and-phototube type of detector, an example of which is described in the patent to Glaude et al, U.S. Pat. No. 3,056,886, a scintillating element, a surface sensitive to alpha particles, is exposed to the atmosphere, the radon content of which is to be determined. The surface emits light scintillations in response to alpha particles. Photomultiplier means, such as a phototube, transforms the scintillations of the detecting surface into detectable electron bursts, and the associated electronic equipment transforms these electron bursts into electric pulses which are counted for a predetermined interval. In a portable instrument the number of phototubes and the power supply are necessarily limited. Also it would be necessary to provide some means to concentrate the radon sample since the planar geometry of the scintillant surface would not be adequate for measuring the radon directly. Any means selected for concentrating the radon must have provisions for removing the radon daughters which build up rapidly and affect the accuracy of the detector. With the plastic scintillantphototube, alpha-type detector, there will be some beta-gamma background, as well as phototube noise, which will raise the sensitivity threshold for radon detection.
The solid-state type of detectors, using small, cryogenicallycooled, solid-state detecting surfaces, present a more serious problem of concentrating the radon-containing gas sample. In a portable solid-state instrument the cryogenic cooling system presents severe size, weight and power requiement problems. Furthermore, the excellent resolution possible with solid-state detectors cannot be fully utilized due to the presence of the radon daughters.
Thus, while many solutions have been proposed for detecting and measuring radon on site, it has not hereto been possible to combine and optimize all of the necessary features for an accurate, quick-responding and portable instrument for on-site radon measurement.
For a portable radon measuring device, the gas porportional detector has a number of advantages over the previously mentioned detectors. The diameter of the counting chamber is limited only by the range of the alpha particle and the length of the counting chamber is limited only by the required portability of the instrument. Thus a counting chamber of sufficient volume to count the radon directly can be of simple design and be of a size and weight below that required for the other types of detectors. At the operating voltage for alpha particles, the proportional detector has an extremely low beta-gamma background and therefore is able to detect radon at lower levels of activity than other known systems. The power supply for the proportional detector is also more stable than that for the scintillant-phototube detector.